Dragging a pointer

[Cruz]

Hi!

I feel the need to drag and drop a pointer to an object (application internally only), but my skills are too weak to accomplish this.

I figure the way to do it is to construct a QByteArray from a pointer to MyObject and put it into a QMimeData object.

Drag:

MyObject *myobject = new MyObject();
 
QMimeData *mimeData = new QMimeData;
mimeData->setData("myobject/pointer", QByteArray( (char*)&myobject, sizeof(myobject) ));

MyObject *myobject = new MyObject();

QMimeData *mimeData = new QMimeData;
mimeData->setData("myobject/pointer", QByteArray( (char*)&myobject, sizeof(myobject) ));

To copy to clipboard, switch view to plain text mode 

This code takes the address of the myobject pointer and converts it to a char pointer. That char pointer is passed as argument to the QByteArray constructor along with the right size, so that the QByteArray should contain the bytes of the address of my object. Right?

Now the unwrapping in the drop:

MyObject* p;
char* data = event->mimeData()->data("myobject/pointer").data();
p = (MyObject*)(*data);
qDebug() << p;
/* BOOM */

MyObject* p;
char* data = event->mimeData()->data("myobject/pointer").data();
p = (MyObject*)(*data);
qDebug() << p;
/* BOOM */

To copy to clipboard, switch view to plain text mode 

mimeData()->data() returns a QByteArray.
QByteArray.data() returns a char* to the data.
So I hoped to take the contents of the data with *data and cast it to a pointer to MyObject, but it crashes.

Little help please?

[spirit]

maybe better keep objects in QMap or in a QHash and pass object id through d&d and then get the pointer from QMap by id.

[wysota]

You are storing a (temporary) pointer to a pointer to an object, not a pointer to an object.

[Cruz]

It works like this:

MyObject* p;
memcpy(&p, event->mimeData()->data("myobject/pointer").data(), sizeof(p));

MyObject* p;
memcpy(&p, event->mimeData()->data("myobject/pointer").data(), sizeof(p));

To copy to clipboard, switch view to plain text mode 

[chezifresh]

How might you accomplish this in python? I'd rather not serialize my objects (which are not QObjects), I want to add mime data for it so I can drag to the desktop and other applications but when dragging internally I really don't want to deal with the overhead.

Is it possible to subclass QDrag, and add a MyDragClass.object() or something along those lines? (requires an isinstance(MyDragClass, dragObject) in lots of places though)